virtualbox.org

I have installed VBox 4.2.18.r88780.
Try to install Win8.1RTM.9600 and it's failed with black screen on boot from ISO mounted. and log see zipped attachment. Command "VBoxManage.exe setextradata w81 VBoxInternal/CPUM/CMPXCHG16B 1" doesn't help.
VT-x/AMD-V acceleration is enabled in BIOS.
But inside Win8 Host a Win8.1 Guest boots good.

VBox.log wrote: 00:00:03.763781 HM: NX not supported by the host
00:00:03.763782 HWACCM: 32-bit guests supported.

Well, it looks like you can't run 64bit guests at all on this host (you neglect to mention what your guest is). Also I vaguely recall that NX support is required by Win8, both 32bit and 64bit versions.

Guest is MICROSOFT.WINDOWS.8.1.RTM.X86.ENGLISH, not x64. See subj too.
What is NX term?

*Vbox on Win8x64 runs Win8.1x64 Guest properly.

You should read and use the Windows 8.1 / 2012 R2 Guest topic instead.

noteirak wrote:You should read and use the Windows 8.1 / 2012 R2 Guest topic instead.

That topic was started 6. Jul 2013 when Win8.1 had been really in preview state.
But now it is RTM.

And yet RTM is not public release, it is still an early release so people can spot bugs & issues. Which is exactly the point of the other topic.

It's release to manufactured! Any change code doesn't allow through next Service Pack been released.

Nevertheless: until the product is available to the public we don't want prerelease sprawl on this site with everyone asking the same FAQ questions as yours. 'Nuff said, topic locked. Please see the stickied Win8 discussions in the Windows Guests forum.